The invention relates to methods of preparing vanadium pentoxide from vanadium-containing solutions, and more particularly from a sodium vanadate solution.
The invention also relates to alumina production since solutions of sodium vanadate are obtained during processing of vanadium-containing sludge in the alumina production.
At the present time used widely are methods of preparing vanadium pentoxide from solutions of sodium vanadate obtained in the processing of vanadium-containing sludge in alumina production, of vanadium-containing titanium-magnetite ore, of phosphorite ore in the production of phosphorus, of carnotite ore in the production of uranium, etc.
Most widely used is the method of preparing vanadium pentoxide from solutions of sodium vanadate that are prepared from vanadium compounds in the corresponding processes, consisting in that solutions of sodium vanadate are processed with ammonium chloride or sulphate at lowered temperatures (20.degree.-30.degree. C.) to precipitate an ammonium-vanadium salt, for example ammonium metavanadate, which is then dissolved in hot water and the obtained solution is processed at a lowered temperature with an ammonium solution. As a result of such processing, an ammonium-vanadate salt, for example ammonium metavanadate, is prepared. As the latter salt is calcined, vanadium pentoxide is prepared.
According to this method, the sodium vanadate solution is processed with the ammonium chloride or sulphate at a pH of 7 to 8.
A disadvantage inherent in this method is a rather high loss of vanadium with wastes due to incomplete precipitation of the ammonium-vanadate salt, in particular ammonium metavanadate, during processing of the starting solution of sodium vanadate with the ammonium chloride or sulphate, and also during subsequent processing of the solution of ammonium-vanadate salt with the ammonium solution.
For example, the vanadium content of the waste solution is 1 - 2.5 g/liter, calculated as V.sub.2 O.sub.5, and the vanadium content of the solution obtained by processing the ammonium-vanadate salt solution (in particular, ammonium metavanadate) with the ammonia solution, is as high as 3-5 g/liter (as V.sub.2 O.sub.5).
Still another disadvantage of this method is the lengthy (10-20 hours) retention of the starting solution of sodium vanadate during its reaction with the ammonium chloride or sulphate which is necessary to attain the above mentioned residual concentrations of vanadium in the wastes.
One more disadvantage of this method is an insufficiently high concentration of vanadium in the solution that is formed by dissolution of the ammonium-vanadate salt in the hot water and processed with the ammonia solution at lowered temperatures. The vanadium content of this solution does not exceed (as experience shows) 40-50 g/liter, calculated with reference to V.sub.2 O.sub.5, since it is limited by the decreasing solubility of the ammonium-vanadate salt, in particular ammonium metavanadate, in water.
The insufficiently high vanadium content of the solution which is decomposed to precipitate vanadium compounds by this or that method is detrimental to the kinetics of the process of isolating vanadium compounds from the solution and precipitation, and requires lengthy retention of the solution of the ammonium-vanadate salt during its processing with the ammonia solution to precipitate the ammonium-vanadate salt. Practical experience has shown that in order to attain the residual content of vanadium of 3-5 g per liter (as V.sub.2 O.sub.5) in the ammonium-vanadate salt solution, (in particular, ammonium metavanadate) after its treatment with the ammonia solution at a lowered temperature, the solution should be kept under the required conditions for 10-20 hours. This, in turn, increases the duration of the entire process for preparing vanadium pentoxide.
Moreover, the insufficient vanadium content of the solution which is formed by dissolution of the ammonium-vanadate salt in the hot water, increased flow rates in the process cycle during further processing of the solutions to isolate vanadium pentoxide. This, in turn, increases energy consumption (heating, cooling, pumping, etc.) and volumes of solutions handled.
A disadvantage of this method lies also with an increased energy consumption for cooling the ammonium-vanadate solution during its processing the ammonia solution to precipitate the ammonium-vanadate salt.
Still another disadvantage of the method is the necessity to recover large amounts of ammonia from the waste gases which are formed during calcining the ammonium-vanadate salt, in particular, ammonium metavanadate, to prepare vanadium pentoxide. The quantity of ammonia (as NH.sub.3) that is liberated per kg of the obtained vanadium pentoxide with the waste gases is 0.187 kg.
The most advantageous method of preparing vanadium pentoxide from solutions of sodium vanadate formed from various vanadium compounds in the corresponding branches of industry, is one consisting in that solutions of sodium vanadate are processed with ammonium chloride or sulphate at lowered temperatures to precipitate ammonium-vanadate salt, in particular, an ammonium metavanadate, which is then dissolved in hot water and the obtained solution of the ammonium vanadate salt is processed, at an elevated temperature with a mineral acid. As a result, a substance containing vanadium, in particular, ammonium polyvanadate is precipitated, which is then calcined to prepare vanadium pentoxide.
The solution of sodium vanadate is processed at a pH of 7 to 8.
This method is characterized by the same disadvantages that are inherent in the widely used method described beforehand, except the high energy consumption and the necessity to recover a large quantity of ammonia.
In the latter method, the solution of the ammonium-vanadate salt, in particular, ammonium metavanadate, is processed with a mineral acid at an elevated temperature. The operation of cooling the ammonium vanadate solution in this method is omitted; hence decreased energy consumption.
In the latter method, the quantity of ammonia in the waste gases of the calcining step decreases. For example, the amount of ammonia during preparation of vanadium pentoxide by calcining the ammonium polyvanadate precipitate is as low as 0.062 kg per kg of the obtained vanadium pentoxide.